This invention relates to a process for removing copper ions from highly acidic waste streams generated during the manufacture of certain dyes or pigments or intermediates thereof by passing the acidic waste streams through a bed of suspended iron particles, preferably a fluidized bed of iron particles. During the process, metallic copper is deposited on the iron.
Various industrial and mining waste waters can contain environmentally unacceptable levels of copper ions. For example, waste streams containing copper can be generated during the manufacture of copper-containing dyes or pigments. Aqueous solutions and sludges containing copper can also be generated in various synthetic processes that use copper, such as the Ullmann reaction, in which aryl halides are coupled to form biphenyls and related compounds; the Sandmeyer reaction, in which aromatic diazonium groups are replaced with halo or cyano groups; and other processes. The amount of copper in these wastes can exceed the levels that can be routinely handled in standard waste treatment facilities. Therefore, efficient, economical removal of copper ions from industrial and mining waste streams has long been sought.
Various techniques for removing copper ions from acidic effluents are theoretically possible and some have been commercialized. Each of the known methods, however, has certain disadvantages that hinder its usefulness for removing copper ions from these effluents. For example, many such methods are useful only for relatively simple idealized systems.
The mining industry, on the other hand, has developed many practical techniques to recover metals that would otherwise be lost. These techniques can often be adapted for use by industries such as the dye and pigment industries, but significant differences between the waste streams exist. For example, industrial waste streams typically contain less copper, more acid (as well as more types of acids), and a higher organic load than metallurgical (e.g., mining) streams. See O.P. Case, "Metallic Recovery from Waste Waters Utilizing Cementation" in Environmental Protection Technology Series EPA-670/2-74-008 (January, 1974). In addition, the organic load, especially for pigments, includes insoluble matter. Thus, those methods used by the mining industry may not always be applicable in more complex systems.
Copper can be removed from acidic solutions by precipitation of insoluble copper-containing compounds. See, for example, F. S. Wartman and A. H. Roberson, "Precipitation of Copper from an Acid Mine Water" in Report of Investigations R.I. 3746, Bureau of Mines, U.S. Department of Interior (1944). For example, the addition of hydroxides or carbonates to such solutions precipitates copper oxides or hydroxides. Copper can even be removed effectively by co-precipitation with ferric hydroxide. Copper can also be precipitated by adding sulfites or sulfides. These processes, however, tend to produce fine solids or gels that can be difficult to filter. In addition, disposal of this solid waste can be economically and environmentally troublesome.
Ionic copper can be reduced to low concentrations by ion exchange methods and can often be recovered from the ion exchange resins in relatively high purity. Ion exchange methods, however, are generally expensive, both in materials and in equipment. Moreover, solids in the effluent can severely hinder their efficiency.
Another method for removing copper from acidic effluents is electrolytic deposition of copper at a cathode. E.g., U.S. Pat. No. 4,152,229. In one variant, a conducting rod is immersed in copper particles that are stirred in such a way that the particles intermittently contact the rod and complete the circuit; the growing copper particles can be removed and replaced as necessary. E.g., U.S. Pat. No. 4,244,795. In this variant, the anode is typically an inert metal. In another variant, the anode is a relatively more electronegative metal such as iron that is converted to an insoluble hydroxide or other derivative. E.g., U.S. Pat. No. 4,280,887. Although electrolysis can be efficient, the method has certain disadvantages. For example, the electrolytic process requires equipment that can be difficult to maintain. In addition, electrolysis can liberate flammable hydrogen gas as a by-product. Not only does this present a safety problem, it represents inefficient use of electricity.
The same general oxidation-reduction reaction that occurs during electrolysis can also be used in a method that does not require an external source of electricity, the so-called cementation method. It has been known for hundreds of years that metallic iron can be used to precipitate copper from aqueous solutions, particularly from the effluent generated during copper mining. H. V. Winchell, "Precipitation of Copper from Mine Waters" (letter) in Mining & Scientific Press. 104, 314 (1912). See also F. S. Wartman and A. H. Roberson, "Precipitation of Copper from an Acid Mine Water" in Report of Investigations R.I. 3746, Bureau of Mines, U.S. Department of Interior (1944); R. M. Nadkarni et al, "A Kinetic Study of Copper Precipitation on Iron--Part I" in Trans. Metallurgical Soc. AIME. 239, 581-585 (1967); and R. M. Nadkarni and M. E. Wadsworth, "A Kinetic Study of Copper Precipitation on Iron Part II" in Trans. Metallurgical Soc. AIME. 239, 1066-1074 (1967). When a solution containing ionic copper is exposed to iron (or another metal that is more electronegative than copper), the ionic copper is reduced and deposited as copper metal while metallic iron is simultaneously oxidized to ferrous iron. Depending on the specific conditions, the ferrous iron is formed as a soluble iron salt or complex or as an insoluble or partly soluble iron compound. Variants of this basic method have used aluminum turnings to remove copper from etchant rinse water in the manufacture of circuit boards (e.g., U.S. Pat. No. 3,905,827) and steel wool to recover silver (e.g., U.S. Pat. No. 4,740,244).
Under acidic conditions, a competing reaction of iron with acid generates hydrogen gas. The competing reaction not only wastefully consumes iron without plating out copper, but, as with the electrolysis methods described above, the hydrogen gas by-product also creates potential safety problems. It has been reported that as a solution is made more acidic than about pH 2 to 3, the rate of copper deposition is almost unchanged but iron consumption increases dramatically. See Wartman and Roberson at pages 3 to 4, and W. W. Fisher and R. D. Groves, "Copper Cementation in a Revolving-Drum Reactor, A Kinetic Study" in Report of Investigations R.I. 8098, Bureau of Mines, U.S. Department of Interior (1976) at page 18; see also Nadkarni and Wadsworth at page 1068. In contrast, the process of the present invention is surprisingly effective at removing copper at much higher concentrations of acid (for example, at a pH less than 1).
Cementation has been carried out using finely divided iron (e.g., powdered or fibrous), particulate or sponge iron, or iron spheres or shot, either batchwise or continuously in columns. See, for example, P. H. Strickland and F. Lawson, "The Cementation of Metals from Dilute Aqueous Solution" in Proc. Aust. Inst. Min. Met., No. 236, 71-79 (1971); A. E. Back, J. Metals, 19 27-29 (1967) (particulate iron); O. P. Case, "Metallic Recovery from Waste Waters Utilizing Cementation" in Environmental Protection Technology Series EPA-670/2-74-008 (January, 1974) at pages 9-22 (shot) and 23 (powder); and K. Kubo et al, J. Chem. Eng. Japan. 12, 495-497 (1979) (spheres). Although often effective in reducing copper levels, the deposition of copper generally makes this method unsuitable for use over extended periods. For example, buildup of copper within the flow channels of a packed bed eventually reduces flow rates and can cause individual pieces of iron to clump or fuse into a mass that can be difficult to remove from the apparatus.
Modified cementation procedures intended to improve the removal of copper have been reported. For example, U.S. Pat. No. 3,766,036 discloses the use of special silicon-metal alloys, including iron-silicon alloys, to remove ionic metallic impurities, such as copper ions, from aqueous solutions. The present invention does not require such exotic materials.
Other modifications of the basic cementation method rely on agitation or stirring. Although stirrers or rotating discs can in theory be used to keep the iron in motion (P. H. Strickland and F. Lawson, "The Cementation of Metals from Dilute Aqueous Solution" in Proc. Aust. Inst. Min. Met., No. 236, 71-79 (1971)), wear and breakage would be expected to reduce their effectiveness.
Tumbling iron nails in a revolving drum reactor has been used to provide agitation during the cementation process. See W. W. Fisher, Hydrometallurgy. 16, 55-67 (1986); and W. W. Fisher and R. D. Groves, "Copper Cementation in a Revolving-Drum Reactor, A Kinetic Study" in Report of Investigations R.I. 8098, Bureau of Mines, U.S. Department of Interior (1976). Although tumbling may inhibit fusion of the nails by keeping them in motion, the references indicate that the primary purpose of the apparatus is to fluidize unattached fragments of copper, which can thus intermittently contact the iron nails and thereby grow in a manner analogous to the electrolytic process discussed above. For the most part, the nails would be expected to remain in contact with one another and, although in motion, would not themselves be fluidized in a way that would maximize the exposed surface area of the iron at any given moment.
A method that fluidizes the iron, on the other hand, might be expected to inhibit fusion of the individual particles of iron. It has been reported that up to 99% of the copper contained in dilute aqueous solutions can be precipitated using particulate iron that has been fluidized in an inverted coneshaped fluidizer. A. E. Back J. Metals, 19. 27-29 (1967), and U.S. Pat. No. 3,154,411. These references disclose use of a pH range of about 2.4 to 3.0, an acidity level in accord with the previously reported optimum range of about pH 2 to 3 discussed above but much less acidic than is used in the process of the present invention.
An object of the present invention was to devise a continuous high-volume process for efficiently reducing copper ion concentrations in highly acidic waste streams formed during the production of dyes or pigments to very low levels using relatively inexpensive reagents. It was a further object to develop a method that produces metallic copper as a by-product that could be recovered for recycling, thereby reducing the generation of unusable solid waste. These objects have been achieved by passing highly acidic waste streams upward through a fluidized bed of iron particles such as chilled iron grit, an irregularly shaped form of iron.